[ORC] Join materialization thread in unit test

There's are race between this thread and the destructor of the test ORC
components on the main threads. I saw flaky failures there in about 4%
of the runs of this unit test.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@328300 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed